At present, the most widespread sealed nickel storage cells are based on the nickel-cadmium (Ni--Cd) couple. They are charged fully at a high rate in two stages. The first stage of charging corresponds to oxidation-reduction of the active materials of the electrodes. Once all of the active material of the positive electrode has been transformed, the storage cell enters an overcharging stage. During this second stage, oxygen is evolved at the positive electrode. Recombination of the oxygen at the negative electrode raises the temperature, and has a secondary effect of lowering the voltage of the storage cell.
To ensure that the cell is fully charged, it is common practice to monitor changes in its voltage or in its temperature, with charging being stopped as soon as the change in voltage becomes negative or the temperature rises.
Recently, new sealed storage cells based on the nickel hydride (Ni--MH) couple have appeared on the market. In numerous applications they are designed to replace Ni--Cd cells. It is therefore essential to be able to use a single charger interchangeably with batteries containing either type of cell.
Unfortunately, the behavior of a nickel hydride cell during rapid charging is different from that of a Ni--Cd cell, in particular with respect to the following two points:
the charging stage is exothermal which means that the cell heats up continuously from the beginning of charging; and PA1 the amplitude of the negative change in voltage on beginning the overcharging stage is much smaller. PA1 Furthermore, present knowledge shows that there is a direct relationship between total overcharged capacity and the lifetime of an Ni--MH cell. Accurate control of charging and minimizing the duration of the overcharging stage are therefore essential for obtaining best lifetime performance. PA1 the change in the temperature of said cells during a charging operation is measured and recorded; PA1 a mathematical expression is established giving temperature as a function of charging time; and PA1 the heating relationship for said cells during said charging operation is stored.
The methods used for detecting the end of charging in Ni--Cd cells are particularly difficult to apply properly with Ni--MH cells because the changes in the observed signals are so small.